The present invention relates to controllers that govern electrically commutated motors, and more particularly to controllers that use back EMF signals to determine rotor position.
Brushless DC motors have been utilized increasingly to replace brushed DC motors in numerous applications, primarily for their advantages of lower cost, higher efficiency, and longer useful life. Electronically commutated motors and drivers generally are provided in two types: sinusoidally commutated motors and trapezoidally commutated motors.
The latter are known as trapezoidally commutated motors because they have a somewhat trapezoidally-shaped back EMF waveform. In a three-phase motor, the phases are driven intermittently and in pairs so that at any given time one of the phases is not driven. This allows the back EMF signal, in particular its zero crossing, to be used to determine rotor position. This configuration is referred to as a sensorless drive.
The motor is driven through selective application of voltages to the different phases in a repeating sequence, i.e. a commutation cycle. FIG. 1 illustrates the six steps of a commutation cycle in a standard brushless DC motor. During each step, two of the phases are in an active state, i.e. either driven at a high voltage or driven at a low voltage, while the third phase is not driven. Between each pair of succeeding steps, two of the phases transition, either from an active state to the inactive state or from the inactive state to one of the active states.
FIG. 2 graphically illustrates the commutation cycle, with motor phases A, B, and C aligned to facilitate recognizing simultaneous transitions. It is to be appreciated that the levels “1,” “−1,” and “0” respectively represent a high voltage, a low voltage (which may be ground), and a center voltage midway between the high and low voltages; in other words, the sum of the high and low voltages divided by two.
FIG. 3 graphically represents the back EMF voltages measured at motor phases A, B, and C over the commutation cycle. Like the applied voltages in FIG. 2, the back EMF voltages in FIG. 3 behave according to the cycle of 360 degrees of electrical rotation, with six steps or segments each with an angular dimension of 60 degrees.
During commutation cycle steps when a given one of the phases is driven, its voltage corresponds to the drive voltage, e.g. positive or negative 1. During segments when the given phase is not driven, the back EMF voltage changes from the voltage applied during the previous segment to the voltage to be applied in the following segment, e.g. as seen in the first cycle segment for phase C in changing from −1 to 1. Further, the change in voltage is substantially linearly related to the change in angular dimension. As a result, the center voltage, i.e. zero voltage in FIG. 3, corresponds to the angular center of each commutation cycle segment.
The correspondence of the zero crossings with cycle segment midpoints is the basis for sensorless commutation of brushless DC motors. Since the zero crossings coincide with the commutation cycle segment midpoints, they can be used to determine starting points for the cycle segments, either in terms of direct angular rotor position or in terms of timed intervals in conjunction with sensing motor rotational speed.
However, it is not always possible to measure the back EMF voltage at the correct zero crossing point. For example, when a given phase moves from a “driven” commutation cycle step during which one of the drive voltages is applied, to an “undriven” segment during which the given phase is not driven, the given phase demagnetizes during the undriven step. At the beginning of the undriven step, the voltage at the given phase includes the back EMF voltage and the voltage generated by demagnetization. The back EMF voltage cannot be measured until demagnetization is complete. Thus, in configurations where the time required for demagnetization exceeds the time equivalent of one-half of the cycle step, it is not possible to directly measure the back EMF voltage at the zero crossing.
Another case in which the back EMF zero crossing may not be measurable directly is when a driving voltage to a given phase is pulsed or chopped. For example, using a half chop to control a driving potential for each phase can result in a back EMF signal measurable only during times when the associated chopped phase is being driven. Unless one of the driven periods coincides with the zero crossing point, the zero crossing cannot be measured directly.
Therefore, the present invention has several aspects directed to one or more of the following objects:                to provide a sensorless drive for a brushless DC motor adapted to synchronize a commutation cycle and the motor, without the need to sense zero crossing of the back EMF signals;        to provide a process for maintaining a commutation cycle synchronized with the position and speed of an electrically commutated motor by measuring back EMF voltages at locations other than the midpoints of commutation steps;        to provide a controller for a DC motor commutation circuit configured to selectively sample back EMF voltages only during times within each commutation cycle step during which the back EMF voltage is directly measurable; and        to provide a process for predetermining off-center locations within the commutation cycle steps for measuring back EMF voltages for a more reliable indication of cycle step center points.        